Applications of the Science

Interest in Allee effects has revived partially as a result of an increasing concern about extinction of rare populations, harvested populations, or pest species. Conservation efforts have clearly benefited from a concern about the existence of Allee effects. When animals are reduced to such low numbers that captive breeding programs are the only way to recover populations, consideration of Allee effects may improve breeding success. In numerous species, breeding is socially facilitated by interactions with other individuals. For some social species like primates, captive breeding efforts that allow individuals to interact with each other may improve breeding success. Furthermore, attempts to reintroduce animals to reserves will likely benefit by accounting for positive effects of neighbors after re-introduction. If social interactions are important either during the settlement process or after individuals have selected habitat, introducing conspecific cues might help establish individuals or groups. For example, ornithologists have had success using decoys to encourage seabirds to settle in unpopulated reserves. Similarly, if Allee effects lead to population clustering, some habitat patches might be packed with residents attracted to conspecifics, while others remain empty despite their suitability. Conservation planners basing reserve selection on the assumption that resident density reflects habitat quality could consequently overlook prime but empty habitats.

A better understanding of the spatial aspects of Allee effects may also help us better manage exploited species. For example, in heavily fished areas of the north Atlantic Ocean, small spawning aggregations of Atlantic cod disappeared, while larger ones remained constant, consistent with population dynamics resulting from conspecific attraction in subdivided populations. Fishing fleets in turn targeted only the larger aggregations, resulting in relatively constant harvest over time even as the number of aggregations declined. Consequently, managers did not detect any decline until the entire north Atlantic population crashed.

Our understanding of invasion biology, and therefore our ability to control outbreaks of pest species, can be improved by taking advantage of potential Allee effects constraining population spread. As indicated above, Allee effects significantly increase the amount of time it takes for a species to establish itself in the core of its range, and significantly decrease the rate at which the population spreads. Examining the mechanisms resulting in growth rates at small population sizes therefore holds promise in improving our ability to control a species once it gains a foothold in alien terrain. For example, some pest managers have manipulated an Allee effect via mate finding to limit spread of pests. Sterile individuals of one sex are released in large numbers into the area containing the pest. This results in difficulties for the opposite sex to find fertile mates, thereby reducing reproduction in the population.

See also: Cooperation; Fishery Models; Metapopulation Models; Social Behavior; Spatial Distribution; Stability.

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